It seems like every time I brew that I am short on OG. I think it may be because I use a 5 gallon carboy and don't mix it well enough before I take the gravity. The closest I have come was when a friend used this siphon to stick in the carboy, deep in it, and I barely missed. Otherwise I use a turkey baster to get wort off the top of the carboy that has had water mixed (which could be another problem).

Here is my brew process:
1. Steep grains in < 1gal of water per 1# of steeping grains.
2. Add water to get up to ~4gallons in the brew pot.
3. Bring to a boil and add half of the DME
4. Boil 60 minutes
5. Add rest of DME
6. Cool
7. Add to carboy and add enough water to bring to 5g.

You have to stir your top off water a ton in order to get a proper reading, which is really hard when you have a glass carboy. I started using part of my auto-siphon to stir the carboy up, as well as general shaking in trying to aerate it. I can now get pretty decent readings.